Abstract

Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all of the main tectonic settings. Most commonly, these systems erupt basaltic magmas within a wide compositional range from strongly silica undersaturated to saturated and oversaturated; less commonly, the spectrum includes more siliceous compositions. Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth's surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to polygenetic systems characterized by relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins. Eruption styles of small-scale volcanoes range from pyroclastic to effusive, and are strongly controlled by the relative influence of the characteristics of the magmatic system and the surface environment.

Small-scale basaltic magmatic systems characteristically occur at the Earth's surface as fields of small monogenetic volcanoes. These volcanoes are the landforms produced by explosive and effusive eruptions triggered by the rise of small batches of magma. Their typical occurrence as volcanic fields is the surface expression of a magmatic plumbing system that is spatially dispersed and episodic. Small-scale basaltic volcanic systems are the most widespread form of magmatism on planet Earth (Cañón-Tapia & Walker 2004) (Fig. 1), although they are also the smallest in terms of erupted magma volume. They are often overlooked in the large-scale purview of plate tectonics, although they occur in all of the major tectonic environments, intraplate, extensional and subduction-related (Cañón-Tapia 2016), providing continuous expression of the interaction between the physical–chemical parameters of the rising magma and the external environmental conditions that influence their eruption styles. The temporal record of monogenetic volcano fields is skewed towards younger epochs because their relatively small volumes render them prone to removal from the terrestrial geological record. Further, they are mostly known from the subaerial record because of the relative inaccessibility of small volcano fields in ocean-floor environments.

Map of younger than Pliocene monogenetic volcanic fields and other important volcanoes extensively studied in recent years or mentioned in this paper. Please note that this collection is not complete. There are numerous less known monogenetic volcanic fields mostly in Central Asia, along the East African rift systems, Ethiopia, along the Andes (mostly in Chile, Colombia and Argentina) and some in the SW USA not listed in this collection due to incomplete information. Please note that this map also does not show volcanic fields associated with large island volcanoes of the Azores, Iceland or Hawaii. Yellow stars refer to volcanic fields (VF); red stars show important polygenetic volcanoes associated with numerous small-volume satelite vents commonly cited in monogenetic volcanism literature (mostly from their morphological aspects); and green stars represent iconic monogenetic volcanoes.

In this paper we review the current state of knowledge of small-scale volcanism from their source in the upper mantle to the eruption characteristics of their surface environment.

The basalt spectrum

Basalts are fundamentally the product of partial melting processes within the Earth, and their compositions are an expression of the temperature and pressure regimes that exist in the outer <150 km of the Earth. Basalt magmas occur in response to a range of environments within this P–T envelope. Chemical compositions displayed by the basalt spectrum range from strongly Si-undersaturated nephelinite (and even more extreme carbonitites) through alkali basalt to silica-saturated tholeiite. The determining parameters of this compositional range are essentially pressure (depth) and temperature (Putirka 2005; Herzberg et al. 2007; Putirka et al. 2007), which in turn constrain the mantle dynamics responsible for melting.

There is a significant positive correlation between the volume of individual magma batches and their position within the basaltic compositional spectrum. Small-volume volcanoes are more alkaline and more Si undersaturated, and larger volume volcanoes are relatively more Si-rich and are less alkaline (McGee et al. 2015). This compositional pattern is readily explained in terms of the ambient parameters within the magma source region, with the nephelinite–tholeiite range representing increasing proportions of partial melting (Fig. 2).

Primary magma produced in equilibrium with residual mantle lithologies is identifiable by virtue of its Mg/Fe ratio (usually expressed as the Mg number ((100×mol MgO)/(mol MgO+mol FeO)) based on the partitioning of Roeder & Emslie 1970). However, truly primary magmas are only rarely erupted at the Earth's surface. Overwhelmingly, the chemical composition of basaltic magma reveals that they have experienced some degree of fractionation, mixing and, in some cases, contamination by cognate or exotic (crustal) material.

The compositions of the magmas that leave the melt production zone in the deeper levels of a system are not easily determined due to the effects of multiple shallow-level processes. This is because most volcanic systems are long-lived zones where rising magmas stall, evolve and interact with preceding batches of magma and their solidified or partially solidified equivalents. Relatively primitive magma compositions are more likely to occur in the small-scale basaltic fields of continental environments, which are characterized by sparse infrequent eruptions, rather than in the persistent highly productive systems established at plate margin and intraplate hotspot centres.

These examples serve to illustrate a fundamental aspect of the spectrum of basaltic magmatic systems: that those with relatively high magma production rates evolve by crystallization and mixing at shallow depths, whereas, in less productive systems, magma stalls within a deeper zone of less permanent storage bodies unless ascent rates are sufficient to overcome the gravitational constraints. Clague (1987) and Clague & Dixon (2000) have shown how this spectrum is expressed by the correlated petrogenesis, petrology and volcanic output rates of Hawaiian lavas as the hot centre of the Hawaiian hotspot is approached, over-ridden and then abandoned by the drifting Pacific lithosphere. In summary, the characteristic feature of small-scale basaltic volcanic systems is the relatively simple and dispersed nature of their plumbing systems. The size, depth and longevity of transport, together with the existence of storage areas beneath a volcano, influence the eruptive patterns and the geophysical and geochemical signals associated with volcanic unrest, as well as the complexity of the petrogenetic history that must be revealed in order to constrain the conditions of melt generation that ultimately drive the volcanic system.

Compositional variations within small-scale volcanoes

A notable feature of many small-scale volcanic cones is the systematic change in the chemical compositions of erupting magma during the course of an eruption (Németh et al. 2003; Smith et al. 2008; Brenna et al. 2010, 2011, 2012a; McGee et al. 2012). Rarely, there are examples of volcanic cones that show no compositional variation. Compositional variation has been observed even in very-small-volume cones and in these the systematic variations are often very clearly correlated with stratigraphy. The general pattern is for material erupted early in a sequence to be relatively evolved and for compositions to become progressively more primitive as an eruption progresses. These patterns have been interpreted as due to fractionation of magmas at high pressures close to their source (Smith et al. 2008). In larger volume cones more complicated compositional patterns have been observed (e.g. Brenna et al. 2010) and interpreted as the result of mixing and mingling of discrete melts in the deeper parts of their system. Compositional discontinuities occurring during the course of a monogenetic eruption sequence have also been observed (McGee et al. 2012, 2013) and interpreted as successive partial melting of distinct, but contiguous, source components with differing melting characteristics in a heterogeneous source.

These marked compositional variations displayed within small-volume magma batches are an important feature of small-scale basaltic volcanoes, and are an indication of their close connection with the high-pressure regions of their respective source regions and the rapidity with which magmas rise from these depths.

The monogenetic concept

Small-volume volcanic cones are usefully termed monogenetic and they characteristically occur in volcanic fields. The terms monogenetic and volcano field are to a degree controversial because their defining parameters are imprecise and depend on the perspective of individual researchers. Both terms have been recently reappraised (Németh & Kereszturi 2015; Cañón-Tapia 2016). Here, we discuss the phenomenon of small-scale volcanism in terms of the linked concepts of monogenetic volcanoes, magma batches and volcano fields (Fig. 3).

Small-volume volcanoes can be classified as monogenetic or polygenetic using two main criteria: (1) petrogenetic features or (2) volcanic architecture. Individually, small volcanoes that are spaced from each other by longer distances than their edifice ‘footprint’ can be recognized easily as small and simple volcanoes that are commonly fed from deep magma sources (a). Such volcanoes over time can form groups, cluster alignments (b) and, if lithospheric conditions permit, some shallower-sourced magmas can also be involved (c). Over time, such dispersed systems can form focused vent clusters (d) and even larger volume longer-lived edifices with multiple vents fed from shallow and deep sources (e). (a)–(e) can be looked at as a spectrum of evolution from a simple monogenetic volcanic field to a more mature field. Individual deep-sourced melts can reach the surface in locations where a complex polygenetic volcano occurs without too much interaction between their magmatic plumbing systems (f). Long-lived stratovolcanoes with multiple shallow magma-feeding systems can provide small-volume eruptions that are clearly shallow sourced but still form simple volcanic edifices which volcanologically would be considered as a monogenetic volcano (g). Long-lived shallow magma-fed systems such as major silicic calderas are commonly associated with a laterally extensive, but internally complicated, network of melt stored at shallow depths (coloured rectangles) similar to proposed models such as those associated with the Okataina Volcanic Complex at the Taupo Volcanic Zone (Smith et al. 2005, 2010; Shane 2015). Such systems can be pierced by deep-sourced melts that then erupt as small monogenetic volcanoes or can trigger silicic melts to erupt and form silicic tuff rings and lava domes (h). In long-lived and large systems, amalgamated compound small-volume silicic volcanic compounds can form with a complex petrogenetic history (i). If lithospheric conditions are favourable, such systems can evolve to more complex and large-volume edifices, losing their dispersed nature both petrogenetically and volcanologically (j).

An established terminology categorizes volcanic systems as monogenetic or polygenetic (Walker 2000). These are useful concepts but suffer from the question of where boundaries that are different in differing areas of investigation may be drawn. In volcanological terms, a monogenetic volcano is one which erupts only once within a defined time period that is recognized as being one in which there is no clear evidence of a temporal break in eruptive activity; the defined time period may be weeks, months, years or, rarely, decades. Observed examples are the scoria cone and associated lava fields of Parícutin (active 1943–52) (Luhr & Simkin 1993; Erlund et al. 2010) or Jorullo (active 1759–74) (Guilbaud et al. 2011) in the central Mexican Volcanic Belt and Mirador, southern Chile (April–May 1979) (Lopez-Escobar & Moreno 1981).

A problem arises because the eruptions of most so-called monogenetic volcanoes were not witnessed, although continuous deposit sequences and the relatively small volumes typical of monogenetic cones strongly support short timescales (Németh 2010; Németh & Kereszturi 2015). In contrast, a polygenetic volcano is one that erupts many times, is fed through an established conduit system that has a relatively long lifespan and delivers discrete eruptive phases separated by clear temporal breaks traceable in the erupted sequence (Manville et al. 2009). In concept, the difference between monogenetic and polygenetic volcanoes is one of plumbing (Fig. 3). In monogenetic systems, batches of magma rise quickly to the surface through simple conduit systems with little interaction with the crustal rocks that they encounter on their way. Polygenetic volcanoes result from plumbing systems that involve the development of magma chambers which show complex interactions with surrounding crustal rocks and extensive evolution through crystal fractionation, magma mixing and magma mingling (Fig. 2). One effect of this contrast in plumbing styles is that the magmas of monogenetic systems are relatively primitive (McGee & Smith 2016), reflecting their compositional connection with their mantle sources, whereas magma of polygenetic volcanoes are more commonly chemically evolved through the operation of assimilation and fractional crystallization in crustal reservoirs.

A further concept which is fundamental to monogenetic volcanoes is that of a batch of magma generated in a discrete melting event and with a defined chemical composition (Fig. 3). Typically, monogenetic eruptive sequences show consistent evolutionary development of geochemical trends which can be interpreted as the evolution of a single batch of magma. Less commonly, a magma batch can show little compositional variation. Some eruptive sequences that are clearly monogenetic in terms of their eruptive behaviour can be shown to represent mixing of magmas of consanguineous, but diverse, origin such as documented from Udo Island, South Korea (Brenna et al. 2010). There are also examples of single volcanic structures that have clearly separated eruptive episodes which produced compositionally discrete batches of magma, as revealed from Rangitoto in the Auckland Volcanic Field (Needham et al. 2011). These cases where a more complex plumbing system can be demonstrated mark the transition between geochemically monogenetic volcanoes and geochemically polygenetic volcanoes, although, from a volcanological perspective, both may be treated as monogenetic (Fig. 3). This is one of the difficulties encountered in applying the term monogenetic.

The distinction between monogenetic and polygenetic is essentially a variable within the spectrum of magmatic systems and one that will have a different definition according to the investigative method. Further, while individual volcanoes in volcano fields can be described as monogenetic in the sense of representing a temporally restricted period of eruptive activity, the systems of which they are a part may have been active for periods as long as those that develop polygenetic volcanoes (Condit et al. 1989; Connor & Conway 2000; Németh 2010). As an example, the polygenetic volcano Ruapehu in New Zealand evolved to its present state over 300 kyr (Gamble et al. 2003), which is equivalent to the total time span of evolution of the Auckland Volcanic Field (Lindsay et al. 2011) that produced in the same time period at least 53 discrete volcanoes (Kereszturi et al. 2013). Monogenetic volcanic fields in some cases can be long lived and overarch entire geological epochs such as those of the Central European Cenozoic Magmatic System (Ulrych et al. 2011) or the Western Arabian Cenozoic Volcanic Province (Moufti et al. 2012).

An important aspect of the debate is the rate of magma supply, which can also be thought of in terms of the connection between source and surface (Fig. 3). The volumes of individual magma batches that produce monogenetic volcanic cones are characteristically small (typically <1 km3, commonly <0.1 km3). The compositional features exhibited by monogenetic magma batches commonly show features that relate to source or near-source processes and these indicate rapid rise rates from source to surface. Because of their small volumes, monogenetic magma batches do not retain a connection to their source and essentially represent ‘bubbles’ of rising magma. If they stall in the crust they become un-eruptible with cooling and crystallization. Recent studies have also indicated that the role of magmas that do not reach the surface and represent ‘failed eruptions’ might be important, even in the case of small-volume magmatism (Gudmundsson 2003; Németh & Martin 2007; Taisne et al. 2011; Geshi et al. 2012; Kiyosugi et al. 2012; Friese et al. 2013; Le Corvec et al. 2013a, b; Cañón-Tapia 2014; Re et al. 2015). Growing evidence has also shown that the shallow plumbing system of small-volume volcanoes commonly defined as monogenetic is complex, and magmas exit through an upper conduit linked to various and geometrically complicated networks of pathways (Valentine & Krogh 2006; Valentine et al. 2007, 2011; Geshi et al. 2011; Hintz & Valentine 2012). Such complex plumbing scenarios add complexity to the chemical and architectural evolution of the volcano, even if they are small in volume (Geshi 2000, 2001). Studies of magma-flow movement within the growing small-volume edifice suggest that magma can behave unexpectedly prior to exiting through a vent, leading to the development of complex conduit–crater networks (Petronis et al. 2013; Delcamp et al. 2014).

Over time, a volcanic system that is typically composed of volcanoes that fulfill the monogenetic plumbing criteria and which shows a high magma production rate can gradually produce compositionally more evolved magmas that feed and build architecturally more complex volcanoes that share similarities to polygenetic volcanoes (Fig. 3). There are numerous examples where a long-lived volcanic field (a ‘mature’ field) can, over time, produce more abundant larger volume, geochemically more evolved compositions and architecturally more complex volcanoes. This has been demonstrated in the Trans-Mexican Volcanic Belt (Aguirre-Diaz et al. 2006; Arce et al. 2013), in the western Arabian Cenozoic Volcanic Province (Camp & Roobol 1989; Camp et al. 1991), in several locations in eastern Africa (Franz et al. 1997, 1999) and in the Chaine des Puys in France (Nowell 2008; van Wyk de Vries et al. 2014).

The edifice-building pyroclastic succession of a single monogenetic volcano reflects the changes of eruptive style during the course of an eruption. Such successions are a key to establishing the timing of eruptive events associated with the edifice (Fig. 3). Especially in larger volume volcanoes, these pyroclastic successions are the subject of debates about the monogenetic v. polygenetic origin of the edifice (McKnight & Williams 1997; Sheth 2014). Normally, in a sensu stricto monogenetic volcano, its pyroclastic succession shows a continuous sequence of tephra layers with no evidence to support significant breaks between pyroclastic beds other than normal changes in pyroclastic density current movement, wind-drifted tephra accumulation or erosional surfaces associated with syndepositional remobilization of pyroclasts due to some other non-volcanic sedimentary processes (e.g. sudden mass movement on the wet flank of a growing tephra ring causing syn-eruptive lahar formation) (White 1991; Manville et al. 2009). In either way, such pyroclastic successions show a consistent sequence of pyroclastic beds associated purely with the fluctuation of the relative role of the internal v. external parameters (Fig. 4). In this context, internal parameters are those that are directly associated with the physicochemical nature (e.g. composition, volatile content, its rise speed and rate, and its changes or variations) of the rising magma, while external parameters are those that can affect the magma fragmentation and, hence, the eruption style and the resulting pyroclastic eruptive products (most commonly associated with the availability of external water of any type and its ability to be available in various timescales). Commonly, there is a trend from a typical phreatomagmatic-fragmentation-dominated eruption style towards more magmatic-fragmentation-dominated successions in the course of the eruption of small-volume volcanoes (Lorenz 1987; White 1991), providing late-stage ‘magmatic cap’ (Fig. 5a), intra-crater scoria cone growth in the initial maar/tuff ring crater (Fig. 5b) or gradually filling the maar/tuff ring craters by lava flows (Fig. 5c). The interplay between the internal and external forces acting upon the eruption style of the growing small-volume volcano commonly form eruptive sequences showing systematic and/or random variations of tephra units associated with the more dominant magma fragmentation styles (Fig. 6). If the eruption takes place as a result of a single eruption of a single magma batch, the pyroclastic successions will show chemical variation patterns associated with either fractionation processes in the tapping magma column en route to the surface or they will show relative compositional homogeneity: such a simple monogenetic volcanic sequence is probably the less common scenario (McGee & Smith 2016).

External v. internal controlling parameters act as ‘competing’ forces to influence magma fragmentation and, hence, the overall architecture of the growing small-volume volcano. On the x-axis of this conceptual diagram, increasing magma volume is shown from right to left and is the fundamental internal force driving volcanic eruptions. On the y-axis, the external forces that influence the magma fragmentation and, hence, the eruption style are marked as the increasing availability (volume) of water to the magmatic system. The external forces can be expressed as a function of external water available, the storage capacity of the aquifers, the hydraulic conductivity, permeability and the surface water availability (a function of climatic conditions). The external forces are heavily dependent on the elevation of the landscape that the magma encounters as higher ground normally has deeper aquifers and/or less potential to capture surface runoff water. In the diagram, two lines separate magmatic-dominated volcanoes (M), mixed-type volcanoes (MIX) and phreatomagmatic-dominated volcanoes (PH). Common types of volcanoes can be distinguished such as: (1) scoria and spatter cones of any size; (2) scoria cones with a thin initial phreatomagmatic base; (3) phreatomagmatic volcanic landform with a thin magmatic cap/infill; (4) well-developed phreatomagmatic landform with a magmatic infill; (5) well-developed phreatomagmatic landform overgrown by a magmatic landform; (6) large well-developed phreatomagmatic landform with a magmatic intra-crater cone; and (7) various sizes of phreatomagmatic landforms. Note that the red arrows represent a shift of the separating lines of volcano types in the case of high magma flux (mfhigh); and orange arrows show the shift of the separating lines of volcano types in the case of low magma flux (mflow).

A typical eruption sequence of a small-volume basaltic volcano (Motukorea/Browns Island, Auckland Volcanic Field) (a) that shows some variation of magma rise (juvenile pyroclast volume changes) and the effect of the external water that influences the eruption style (accidental lithic contents). The section is dominated by an initial phreatomagmatic sequence (ph) interbedded with a magmatic-fragmentation-dominated unit (m). In the section, ballistic bombs of accidental lithic fragments caused impact sags (arrows). The entire section is capped by a magmatic capping unit dominated by scoriaceous successions (mc). In addition, random or systematic changes in the upper conduit can trigger vent-clearing events that are commonly associated with some chemical changes as a reflection of the arrival of a new melt batch below (a). A common trend in a small-volume volcanic eruption that finishes with the development of an intra-crater scoria cone (b), such as in Meke Gölü (Meke Lake) in Anatolia's Karapinar Volcanic Field, or lava spatter cone growth with intra-crater lava infill, such as La Breña maar in the Durango Volcanic Field in Mexico (c).

Small-volume cones can be heavily influenced by phreatomagmatism such as the Ohakune cone complex near Ruapehu New Zealand. Lighter coloured beds have a strong phreatomagmatic influence, while darker beds are dominated by lava spatters; many clasts have chilled rims, indicating that lava fountaining took place through a wet environment. Numbers represents individual units identified by Kósik et al. (2016).

Most commonly in a small-volume volcanic edifice, the pyroclastic succession shows few distinct horizons commonly marked by ballistic bomb and block layers. These layers can be traced over large areas, and seem to be associated with a volcanic explosive event that was dispersed equally in every direction within a very narrow time period and represent materials that are derived from conduit walls and/or from a degassed magma stalled in the upper conduit or crater. These coarse-grained horizons are typically associated with random conduit collapse events but they also can be associated with a systematic movement of the explosion locus along a fissure (Sohn & Chough 1989; White & Ross 2011; Graettinger et al. 2015). In either way, the presence of such horizons can reflect conduit dynamic processes along and across the feeding dyke involved in the eruption (Ross & White 2006; Barnett et al. 2011; Geshi & Oikawa 2014). Commonly, such horizons can also be associated with a slight change in the chemical compositions of the juvenile pyroclasts above such a horizon, reflecting the arrival of a new magma batch that triggered the excavation (Brenna et al. 2011, 2015). In such cases, the initial explosion breccia horizons can contain evidence of syn-eruptive erosion, mud draping or clast rearrangement on the millimetre–decimetre scale, or even erosion events, as in falls of condensed water onto the growing edifice flanks.

Because feeding dykes are commonly blade-like in form and follow fissures (as demonstrated by the presence of a row of craters in many settings), the lithological and, hence, the hydrogeological variations along a fissure (over length scales of hundreds of metres) can cause significant lateral variations of eruption style and strikingly different eruptive products along the fissure, which provides an ‘impression’ that the growing volcanic edifice is complex and has departed from a sensu stricto ‘monogenetic’ nature (Fig. 6). In small magmatic volumes, the relative influence of external parameters on the resulting volcanic edifice structure and their pyroclastic succession can be characteristic (Fig. 7). For larger magma volumes, the effect can be difficult to identify as later eruptive products may completely cover any sign of earlier events (Fig. 7).

Initial phreatomagmatic layers in a very-small-volume scoria cone in the AD 641 Al Madinah eruption in Saudi Arabia as a sign of initial favourable conditions of external water to influence the intruding magma tip during the onset of the eruption.

In other more complex systems, the presence of a range of basaltic compositions and of intermediate and felsic compositions (e.g. benmoreite, trachyte, phonolite) is evidence of compositional modification during transit to the surface.

The tectonic setting, as well as the local or regional stress field, appear to be important factors in the genesis and form of some volcanic fields (Le Corvec et al. 2013a), and in fact may govern whether polygenetic or monogenetic structures are formed (Takada 1994; Bucchi et al. 2015). The position of the volcanic arc in Mexico is thought to cause the shift from polygenetic to monogenetic volcanism from north to south (Connor 1987). The evolution of some volcanic fields has been linked to tectonic plate movement, such as in the San Francisco Volcanic Field (Arizona) where volcanism is thought to migrate with the westwards movement of the North American Plate (Tanaka et al. 1986). In other cases, small-volume dispersed volcanism shows random distribution with no obvious trend associated with inferred plate motions. However, their occurrence is more likely to be associated with sudden changes in the ‘topography’ of the lithosphere asthenosphere boundary, as has been demonstrated from the Pannonian Basin in central Europe (Harangi et al. 2015). Tectonic setting and structure has also been implicated as the cause of small-volume magmatism in some volcanic fields, such as the edge-driven convection model of Demidjuk et al. (2007) for the Newer Volcanic Province (SE Australia) where a lithospheric step is thought to cause the upwelling required to stimulate melting. A similar idea to this is invoked for the Zuni-Bandera Volcanic Field of New Mexico, where changes in lithospheric thickness beneath the field are linked to changes in the melting processes (Peters et al. 2008). On a local scale, the distribution of volcanic cones within a field can reflect the orientation of faults (Muffler et al. 2011).

The geochemical character of monogenetic volcano fields is in part related to their specific tectonic setting, and includes intraplate (e.g. eastern Australia, western North America and northern New Zealand), extension (e.g. central Europe and Arabia) and subduction-associated (e.g. Mexico, USA, Argentina) settings. Important underlying factors for the development of monogenetic volcanic systems are small magma volumes, episodic eruption of discrete magma batches and a crustal environment that allows the passage and escape of small magma volumes through the crust.

Volcanic fields can form in a range of surface areas from few tens of square kilometres to over 1000 km2 area, within which there may be a few to more than a hundred volcanoes (Connor & Conway 2000). The vent distribution in a dispersed volcanic field has been a common subject of studies intended to establish a temporal–spatial vent evolution concept within a single volcanic field (Le Corvec et al. 2013b). There are volcanic fields in which vents show a marked alignment normally associated with faults such as the Chaine des Puys in France (Boivin & Thouret 2014; Lutz 2014). The vent alignments can, hence, reflect older underlying structural elements that might have been rejuvenated in the course of the volcanism, as argued for many cases in vent distributions over old continental lithospheric regions (Mazzarini & D'Orazio 2003). There are volcanic fields where vents instead form clusters, and it has been suggested that they represent the surface expressions of narrow ‘mantle fingers’ (Tamura et al. 2009), and there are volcanic fields where vents show random distribution and where any link to structural elements is difficult to establish (Condit et al. 1989).

There are very few systematic studies which connect the magmatic plumbing system of silicic magmas and their volcanic architecture in a monogenetic context (Brenna et al. 2012b; Ridolfi et al. 2016). This is partially because small and short-lived silicic volcanoes are commonly associated with large and long-lived volcanic systems, and commonly represent a less important fraction of the total system. This concept, however, needs some revision as small monogenetic silicic volcanoes are more common and their eruption occurrence more frequent than is generally appreciated, and they can play an important role in the overall volcanic hazard scape in large and complex volcanic systems. Small silicic volcanoes that form lava domes and/or small explosion craters (maars or just small silicic edifices) are common features in association with large caldera networks, such as those of the Taupo Volcanic Zone in New Zealand (Houghton et al. 1991; Cole et al. 2010, 2014). Among these small volcanoes, some are very young ones and show clear evidence of short eruption durations and are well below the 1 km3 eruptive volume (such as the 14 kyr-old Puketerata tuff ring and dome near Taupo in New Zealand: Fig. 8d) commonly used as a proxy to argue for their monogenetic nature (Brooker et al. 1993; Stevenson et al. 1994; Druitt et al. 1995; Kazanci et al. 1995; Bursik et al. 2014; Dennen et al. 2014; Moufti & Németh 2014).

There are at least two major groups of non-basaltic volcanoes that can be viewed as monogenetic: (1) volcanoes that erupt in continental settings through thick continental crust, such as those in western Arabia (Camp et al. 1991); and (2) volcanoes that, in some degree, show an association with a shallow magma source feeding large-volume, commonly caldera volcanism, such as the Long Valley Caldera (Hildreth 2004) (Fig. 6). A typical scenario for this later group of small volcanoes are those that are fed by a small-volume melts released between major caldera-forming events and inferred to be fed from the same major magmatic sources associated with the main complex and polygenetic volcanic network. Those volcanoes that clearly show an individual feeding network that taps the deeper zones of a magmatic system seem to be associated with volcanic fields that were active over a long time, allowing the capture of magma in the thick crust and its evolution to more silicic compositions. Here, we suggest that in spite of the different magmatic plumbing system associated with these volcanoes, the result on the surface can be very similar in terms of their volcanic architecture. The separation of these volcanoes from other volcanoes with long-lived and stabile feeding systems is important not only from a volcanic hazard perspective in young volcanic regions, but also from mineral exploration aspects in older settings where the edifices might be largely removed due to erosion and there is access to their upper conduit zones commonly associated with mineralization. The facies architecture of such exposed plumbing systems of non-basaltic monogenetic volcanism is the key to understanding the overall magma-release processes through individual magma-feeding networks that operate under small supply volume conditions. This situation is clearly different from those systems that have a driving mechanism associated with a larger volume magma supply, and a broader and more interconnected plumbing network that can retain heat long enough to generate heat to drive a geothermal mineralization system.

Eruption style variation: the ‘competition’ between the magmatic system and the environment

The style of volcanic eruptions in small-volume monogenetic volcanic fields is strongly dependent on the relative influence of internal magmatic (e.g. magmatic volatiles, chemical composition and viscosity) and environmental factors (e.g. the presence of external water, host sediment physical conditions and fractures) (Németh 2010; Németh & Kereszturi 2015). Essentially, this can be expressed as a ‘competition’ between the magmatic system and the near-surface environment encountered by the rising magma (Fig. 4). In most cases, the volumes of magma batches that feed monogenetic volcanoes are well below 1 km3 (closer to 0.01 km3), and the balance between magmatic and environmental factors can be very sensitive (Kereszturi et al. 2013, 2014).

In a very simplified model, if magmatic volumes are larger (i.e. increasing heat and potential energy ‘stored’ in the rising magma), the system can overwhelm the external environment to produce a dominantly magmatic eruption, and typically Hawaiian–Strombolian eruption styles constructing spatter and scoria cones (Kereszturi & Németh 2012a; Kereszturi et al. 2014). For smaller magma volumes, and potentially lower magmatic flux and therefore eruption rates, external environmental factors will dominate the course of the volcanic eruptions to produce phreatomagmatic eruption styles and associated pyroclastic deposits (Kereszturi et al. 2014). The systematic nature of these processes is commonly observed in the basal pyroclastic succession of monogenetic cones (Fig. 5a). In the initial pyroclastic succession of cones that were produced in an environment where a minimal amount of external water (surface or fracture stored) was available, a thin (metre scale as a maximum) phreatomagmatic pyroclastic deposit always appeared (Murcia et al. 2015) (Fig. 7). Similarly, if the magma supply rate drops, a short-lived phreatomagmatic blast can produce a thin pyroclastic deposit indicating that the eruption style has changed due to changes in the magma rise rate and the access of external water to the rising magma. These processes can leave a dominant textural feature in the pyroclastic succession that may give an impression of major changes in the eruption; however, these changes were caused only by the subtle interaction between the external and internal controlling parameters of the eruption.

Changes in the magma rise rate, magma volume and environmental conditions causes changes in eruption style, leading to cyclical activity patterns. Such trends have recently been documented in a number of volcanic fields (Martin & Németh 2005; van Otterloo et al. 2013; Agustin-Flores et al. 2014, 2015). For example, in the Auckland Volcanic Field in New Zealand, magma volumes vary by orders of magnitude (0.01–1 km2) and there are widely variable conditions of water availability, and as a result there is a wide range of eruption styles in contrast to volcano fields which occur in relatively dry conditions or where magma volumes are larger (Kereszturi et al. 2014). Low magma volumes and the availability of near-surface water in the Auckland Volcanic Field have played a major part in determining eruption styles, and as a consequence about 75% of the volcanic cones were initiated by a significant explosive phreatomagmatic eruptive phase (Kereszturi et al. 2014). Recent studies imply that the relative influence of the magma system and environmental factors can be calculated and integrated into a relatively simple numerical model that can be viewed as the eruption style formula of this specific field (Kereszturi et al. 2017); similar expressions can be derived for other monogenetic magmatic systems.

The environmental influence on a monogenetic volcanic eruption can fundamentally change the potential volcanic hazard from a relatively moderate explosive eruption style that can require a particular type of response to a more violent, phreatomagmatic style that requires a quite different response (Lorenz 2007; Németh et al. 2012).

The interplay between the internal v. external parameters that influence the eruption style of small-volume volcanoes can also vary over longer time periods. Typical volcanic fields with more than a dozen volcanic edifices commonly formed over tens of thousands to millions of years, such as the Wudalianchi in NE China which has 14 volcanoes in the past 2.1 myr (Gao et al. 2013). However, there are also volcanic fields that formed over tens of millions of years but in these there are generally clearly defined periods of eruptive activity: for example, many of the mature volcanic fields in the Arabian Peninsula (Camp & Roobol 1992; Moufti et al. 2012) or in Australia (Boyce 2013).

The consequences of a long lifespan of a monogenetic volcanic field is that the eruption styles preserved in the geological record can carry important information on the environmental conditions that prevailed during the time frame of the field. Climate changes can provoke changes in the surface and subsurface hydrogeology of a region, and this is one of the single most important external factors that can influence the eruptions style. For example, this has been demonstrated in the Bakony–Balaton Highland Volcanic Field in Hungary, a basaltic intraplate field that produced at least 35 volcanic edifices over a nearly 6 myr time period between 8 and 2.3 Ma (Wijbrans et al. 2007); volcanoes dominated by phreatomagmatism are clearly more abundant at a time when palaeoclimatic data indicate more humid and wet periods (Kereszturi et al. 2011). Similar trends have also been suggested from the Trans-Mexican Volcanic Belt (Siebe 1986) and from the Arabian Peninsula (Moufti et al. 2015). While these ideas are logical, so far no systematic studies have been carried out in other fields with longer time spans.

Similarly several studies have demonstrated a potential link between wet periods characterized by saturated subsurface aquifer conditions and more environmentally dominated eruption styles (Siebe & Salinas 2014; Kshirsagar et al. 2015, 2016). Some workers have suggested the influence of large pluvial or inland lakes where basaltic magma rise is ongoing over millions of years. Such a situation has been demonstrated along the western Snake River, where shallow subaqueous volcanoes formed along the margins of a large inland lake (Godchaux & Bonnichsen 2002). With a reduction in the surface area of the lake, the younger Surtseyan volcanoes tend to be confined more towards the present-day axis of the modern western Snake River (Godchaux et al. 1992; Brand & White 2007). Palaeolake-level changes and their influence on the eruption styles of rising magma has also be recorded along Lake Kivu, where the location of Surtseyan and phreatomagmatic volcanoes seems to correlate well with the changing location of the palaeoshoreline of the lake (Capaccioni et al. 2003; Ross et al. 2014, 2015). Similar examples have been reported from Anatolia (Keller 1975) and in several intra-mountain basins in the Basin and Range region of the western USA (White 1990, 1996). It is very likely that the influence of large lacustrine basin evolutions in Central Asia, North Africa and across the Arabian Peninsula influenced the style of volcanism: however, so far, systematic studies have not been performed in this regard.

Volcanosedimentary response and preservation potential

Monogenetic volcanic fields are composed of individual small-volume volcanic edifices, each with a relatively simple upper conduit–crater–vent system, that are normally spaced from each other at distances longer than their edifice base diameter. The small volume of magma involved and the variable external conditions influence the style of eruption and determine the variety of associated eruptive deposits. Where magma volumes are low and environmental conditions wet, phreatomagmatic eruption styles may prevail over the entire duration of the growth of a single volcano. Such explosive eruptions are expected to produce tephra deposits extending over several tens of kilometres from their source. In spite of the low magma volume, such volcanoes can produce reasonable-sized volcanoes because of the relatively large volume of country rock that is ‘recycled’ as non-volcanic pyroclasts (Németh et al. 2012). In such environmentally controlled eruptions, the ‘footprint’ of each cone is relatively large and tephra deposits relatively extensive (Németh et al. 2012). While these tephra blankets are normally thin and their preservation potential low, the volcanic field will be dominated by large numbers of depressions (craters) that then can act as small sedimentary basins to ‘harvest’ ash from other sources (White 1991). Such volcanic fields can quickly lose their volcanic appearance due to vegetation cover and extensive erosion of the relatively small volcanic edifices. If a volcanic field is active over a long time and the magma production rate large, the sedimentary contribution to the terrestrial record can be significant and may be preserved over a longer time as part of the continental sedimentary successions (Manville et al. 2009; Martin-Serrano et al. 2009).

The preservation potential of the volcanic eruptive products of phreatomagmatic-dominated volcanic fields is unknown in the long term. While craters are excellent sites to host tephra records, commonly the only record of the existence of such volcanic fields in the geological past is the exposed diatreme associated with maar volcanoes. The information that can be gained from diatremes is, however, restricted to the understanding of the individual volcano and cannot be used for correlative purposes to refine the eruption history of the volcanic field as a whole (White & Ross 2011). In addition, because diatremes are pyroclast-accumulation zones where individual explosive events excavated and recycled pyroclasts, it is a significant challenge to establish the original eruptive volume of the volcano and, hence, establish its fundamentally monogenetic origin (White & Ross 2011). This problem has recently been demonstrated through careful examination of some kimberlite-bearing diatremes and other mafic diatremes (Kurszlaukis & Fulop 2013; Fulop & Kurszlaukis 2014). In eastern Germany, some diatremes recorded eruptive products found to be millions of years apart, apparently hosted in the same narrow and well-defined pipe-like features normally interpreted to be the result of a single monogenetic volcanic eruption under wet – environmentally controlled – conditions (Suhr & Goth 2009; Buechner et al. 2015). A similar scenario has also been recorded in several kimberlite pipes, where clear geochemical, age and textural evidence showed that a single pipe can host multiple ‘zones’ formed in separate events in a kind of a compound monogenetic scenario (Kurszlaukis & Barnett 2003; Barnett 2008). It seems that, for some reason, these pipes functioned as volcanic conduits for successive monogenetic eruptive events separated by long time periods, commonly reaching the range of the total lifespan of the entire volcanic field they belong to.

It appears that there are a large number of phreatomagmatic-dominant monogenetic volcanic fields, especially in coastal areas, large intra-continental lacustrine basins or just in well-drained areas with a good groundwater network. Although such conditions favour phreatomagmatism in the evolution of the volcanic field, it is rare that there is no variation in the eruption style from volcano to volcano across the field. If the volcanic field operates with elevated magma output rates and potentially higher magma flux rates, most of the volcanoes of the field, even if they have dominantly phreatomagmatic early phases, reach purely magmatic explosive and/or effusive conditions in their later stages. such as has been demonstrated from the Auckland Volcanic Field in New Zealand (Kereszturi et al. 2014).

Volcanic fields where the eruptions are dominated by magmatic (dry) conditions are composed of numerous scoria and spatter cones, and lava flows and fields. The pyroclast-preservation potential of such fields can be long in arid conditions; however, in humid climates, such cones can vanish over tens of thousands of years. It is inferred that degradation of scoria cones follows a regular pattern: hence, such cones can be used for relative age dating (Wood 1980; Fornaciai et al. 2012). However, recent studies demonstrated that such methods need to be treated carefully as the syn-eruptive processes might have a larger impact on the cone architecture and their degradation than it is commonly thought (Kereszturi et al. 2012; Kereszturi & Németh 2012b).

Conclusion

The concept of monogenetic and polygenetic volcanoes is usefully applied to the spectrum of volcanic systems from small to large. Monogenetic volcanoes are defined by small magma volumes, short eruptive periods and dispersed plumbing, although the magmatic systems to which they belong may be long lived. The characteristic expression of monogenetic volcanic systems is as fields of small volcanic cones.

An important feature of basaltic monogenetic volcanic systems is that observed patterns of compositional variation are commonly linked to differentiation processes that have occurred at high pressures close to their sources or to differential partial melting in their mantle sources. This illustrates the close link between magma sources and the eruption of magma at the Earth's surface, which points to rapid rise rates and little interaction with the rocks through which the magma rises – an important distinction from polygenetic systems.

There is a clear separation of monogenetic systems from the basalt-dominated volcano fields which are linked to deep hot mantle sources and fields where there are significant amounts of evolved compositions that are related to processes operating at crustal depths. Small-volume volcanoes with a significant proportion of evolved compositions are related to systems which have a high magma supply rate where magmas have stalled within the crust and fractionation processes have led to the evolved compositions. These volcanoes represent a transition towards the complex edifices that characterize polygenetic volcanic systems.

An important concept that links the nature of the magmatic system to the environment in which magmas erupt at the Earth's surface is one of a competition between the rising magma and the nature of the eruptive environment. Where the system dominates, magmatic eruption styles (Hawaiian, Strombolian, effusive) create scoria cones and lava flows in what can be termed ‘dry’ conditions. In contrast, where the environment dominates and the availability of water profoundly influences the behaviour of erupting magma, eruption styles are dominated by phreatomagmatism, and the production of tuff cones, tuff rings and maars.

Small-scale magmatic systems, commonly expressed at the surface of the Earth, represent the rise of small-volume batches of magma into spatially restricted domains. Although individual magma batches are small, this is the most widespread form of volcanism on Earth. The monogenetic volcanoes which are a feature of such systems provide a unique window into processes in the upper mantle that give rise to magmas. Further understanding their behaviour underpins hazard scenarios where human activities impinge on their existence.

A snapshot of current advances in research on monogenetic volcanism

Small-scale volcanic systems expressed at the Earth's surface as fields of small volcanoes are the most widespread form of volcanic activity on the planet. Because individual volcanoes in these fields are typically formed during a temporally restricted period of time, the term monogenetic has become a useful descriptor for this type of volcanism, although it is not one that is universally accepted. Monogenetic volcanism has received a lot of attention in recent years, partly because the small scale of their associated magmatic systems enables the preservation of unique petrological features and provides a ‘window’ into the processes that produce their magmas, because the details of their volcanic processes are readily interpreted and also because, despite their small scale, there is a realization that many communities worldwide are vulnerable to the effects of future volcanic activity.

This Special Publication has arisen from the activities and discussions at workshops and conferences during recent years, including the International Maar Conferences, the commemorative 250 year anniversary on the Jorullo scoria cone eruption, various thematic sessions on monogenetic volcanism offered in major volcanological congresses such as the International Association of Volcanology and Chemistry of the Earth's Interior Scientific Assembly, the General Assemblies of the International Union of Geodesy and Geophysics, the Geomorphological World Congresses, the American Geophysical Union meetings, and several regional workshops. This volume is not intended as a comprehensive volume on the nature of monogenetic volcanism but, rather, is a snapshot of the current state of research into this important type of volcanic activity. The diverse nature of research into monogenetic volcanism during the past decade, together with the far-reaching outcomes that have resulted, demonstrates that a unified definition and understanding of the processes that drive monogenetic volcanism is not yet available.

In this introductory chapter we have reviewed the current state of understanding of the chemistry and volcanology of monogenetic volcanic fields. The following chapters deal mainly with the volcanological aspects of monogenetic volcanism, the way that volcanic cones grow through various eruptive processes (Bemis & Ferencz 2017; Lorenz et al. 2016) and the relationships that these have to the immediate underlying conduit (Kurszlaukis & Lorenz 2016).

An important aspect of the study of monogenetic volcanic systems has been the way that an understanding of their behaviour has been built on detailed studies of systems in widely dispersed localities and in a wide variety of geological and tectonic environments. Much of this volume has been devoted to presenting the current perspective of volcanism in different parts of the world. Cas et al. (2016) and Murcia et al. (2016) describe regional-scale studies of the western Victorian (Australia) province and the northern part of Harrat Rahat in Saudi Arabia. Fulop & Kurszlaukis (2016) present the results of a study of a kimberlite pipe in Ontario, highlighting the complexity of a kimberlite pipe reflecting the potential rejuvenation of volcanism in the exact same location and producing texturally and chemically complex diatremes. There follows chapters on several small-scale monogenetic fields in Mexico (Alvarez et al. 2017a, b; Aranda-Gómez et al. 2016; Saucedo et al. 2017), Argentina (Báez et al. 2016; Maro & Caffe 2016b) and Colombia (Borrero et al. 2016). These serve as an illustration of the importance of individual studies in different settings from around the world.

Acknowledgments

This paper is result of discussions with many colleagues across the globe. Discussion sessions during the past and recent International Maar conferences were particularly stimulating. Many aspects of the researches resulted in this review were funded by various agencies such as the Massey University Research Funds (2016), and New Zealand National Hazard Platform Project. Reviewers’ comments by Xavier Bolos and Philip T. Leat were greatly appreciated.

BucchiF., LaraL.E. & GutierrezF.2015. The Carran-Los Venados volcanic field and its relationship with coeval and nearby polygenetic volcanism in an intra-arc setting. Journal of Volcanology and Geothermal Research, 308, 70–81.

KereszturiG., NémethK., CsillagG., BaloghK. & KovácsJ.2011. The role of external environmental factors in changing eruption styles of monogenetic volcanoes in a Mio/Pleistocene continental volcanic field in western Hungary. Journal of Volcanology and Geothermal Research, 201, 227–240.

MartinU. & NémethK.2005. Eruptive and depositional history of a Pliocene tuff ring that developed in a fluvio-lacustrine basin: Kissomlyo volcano (western Hungary). Journal of Volcanology and Geothermal Research, 147, 342–356.

NémethK. & MartinU.2007. Shallow sill and dyke complex in western Hungary as a possible feeding system of phreatomagmatic volcanoes in ‘soft-rock’ environment. Journal of Volcanology and Geothermal Research, 159, 138–152.

SiebeC.1986. On the possible use of cinder cones and maars as paleoclimatic indicators in the closed basin of Serdan-Oriental, Puebla, Mexico. Journal of Volcanology and Geothermal Research, 28, 397–400.

SperaF.J.1984. Carbon dioxide in petrogenesis III: role of volatiles in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas. Contributions to Mineralogy and Petrology, 88, 217–232.